In measuring gravity, it is first defined as a vector force between the earth and a mass which is attracted to the earth. Being a vector, gravity has three components defined along mutually perpendicular directions. Co-pending U.S. patent application Ser. No. 083,351 filed Oct. 10, 1979 is directed to an apparatus which measures one of the component vectors. This disclosure is directed to an apparatus which measures the horizontal component of gravity. By defining the coordinate system such that the major component of gravity is a vertical component along the Y-axis, there still remains a pair of horizontal components which are mutually perpendicular to one another. Assigning an arbitrary dimension in conformance with well known standards, they can easily be described as the north-south component which is perpendicular to an east-west component. For the sake of brevity, these will be referred to hereinafter as the north and east components. They are measured by the same apparatus, the apparatus being simply rotated to a different axis of rotation for the equipment.
It is perhaps somewhat simplistic to state that gravity is solely a component force acting on a mass from the center of the earth. The center of the earth is the dominant force in terms of magnitude. Gravity, however, is a three-dimensional vector. It is formed by the addition of vector forces acting on a mass from other bodies more remote than the earth. As will be recognized, the attractive force of the earth is quite large compared to other forces, but they, nevertheless, exist. Other celestial bodies contribute forces which are added as vectors so that the net force acting on a test instrument or mass of interest is the resultant vector from all of these celestial bodies acting together. This is a recognized fact from time immemorial in observation of the tides which are influenced by the moon. Thus, the moon contributes to the net gravity force acting at a given point.
This disclosure is directed to a gravity instrument which records horizontal components of gravity. The horizontal components are thus the two remaining components of the three-dimensional gravity vector which acts on a body. It has been discovered that the horizontal components are reasonably significant in geological prospecting. Consider the instance where a large metallic ore body near the surface of the earth is mapped. A vertical component gravity measuring instrument of the sort disclosed in co-pending U.S. patent application Ser. No. 083,351 filed Oct. 10, 1979 can be moved over the ore body to detect anomolies in a gravity map which are indicative of the ore body. It has been discovered, however, that mapping horizontal components of gravity at points to the side of the ore body will also indicate the presence of the ore body. In other words, the anomoly of a large ore body alters measurements in gravity in all dimensions. Needless to say, there is a scale factor involved which is dependent on the closeness of the ore body, the mass of the ore body, the specific gravity of the ore within the ore body and other scale factors. However, it is important to note that the ore body anomoly is not limited solely to vertical components of gravity.
The present apparatus is a structure which detects variations in the horizontal components of gravity. The apparatus accomplishes this result by first nulling the apparatus so that the center of gravity is on a line with the axis of rotation of the equipment. The axis of rotation is adjusted as a preliminary step to a horizontal position. Horizontal can be determined as, for instance, in the use of a bubble level. By so doing, the vertical component of gravity acts merely to keep the equipment aligned with the horizontal. The equipment, having an extended lower arm which points toward the center of the earth, is thereafter able to rotate with variations in horizontal gravity. In other words, variations in horizontal gravity are sensed by the balanced lower arm. The lower arm is closer to the center of mass of the earth and is, therefore, more susceptible to gravity force variations. As variations occur, the lower vertical arm is attracted in a measure of horizontal components of gravity. The angular deflection of the lower arm is proportionate to changes in gravity.